The present invention relates to a digital amplifier suitable for an audio amplifier or the like.
There is a digital amplifier which generates a pulse train subjected to pulse width modulation in accordance with an input signal, and switches a switching amplifier stage inserted between a power supply and a speaker based on the pulse train so as to drive the speaker. In such a digital amplifier, a power supply voltage fluctuates due to the switching operation of the switching amplifier stage so that the gain fluctuates. Without any countermeasure, waveform distortion occurs in the drive waveform of the speaker due to the fluctuation of the power supply voltage. In order to prevent such waveform distortion from occurring, a related digital amplifier detects the waveform of the power supply voltage applied to the switching amplifier stage, and makes control to keep the gain constant on the basis of the detected waveform. Such a digital amplifier is, for example, disclosed in Japanese Patent Laid-Open No. 2004-7324 (Patent Document 1).
In the related digital amplifier, the power supply voltage waveform is fed back directly to a control circuit for keeping the gain constant. Therefore, a signal component unnecessary for control to keep the gain constant, such as switching noise included in the power supply voltage waveform, is also used for the gain control. Thus, there is a problem that sound reproduction may be adversely affected.